1. Field of the Invention
This invention relates to a new sub-denier spunbonded nonwoven web product produced by a unique spunbond apparatus and its unique operating process for the continuous production of thermoplastic synthetic resin filaments at unusually high filament speeds. More particularly the invention relates to the production of such nonwoven webs by this spunbond apparatus utilizing extremely high fiber speeds, generally of the order of 80 m/sec and more typically exceeding 100 m/sec. resulting in fibers on the order of 1.0 denier and less. In another important aspect, the invention relates to a nonwoven fabric possessing a more uniformly random web structure with sub-denier fibers created by the inventive apparatus and method. This web structure results in a narrower ratio of machine direction to cross direction tensile properties in addition to significantly improved cover and greater opacity.
2. Prior Art
It is well known to produce nonwoven webs from thermoplastic materials by extruding the thermoplastic material through a spinneret and drawing the extruded material into filaments by eduction to form a random web on a collecting surface. U.S. Pat. No. 3,802,817 to Matsuki et al describes a full width eductor device and method which requires high pressures, however it is limited to lower speeds for practical operation. U.S. Pat. No. 4,064,605 to Akiyama et al similarly describes apparatus employing high speed air jet drafting with the same inherent limitations. U.S. Pat. No. 5,292,239 to Zeldin et al discloses a device that significantly reduces turbulence in the fluid flow in order to uniformly and consistently apply the drawing force to the filaments, which results in a uniform and predictable draw of the filaments. This system limits the magnitude of attenuation because of insufficient draw forces due to the extremely shallow jet angle. U.S. Pat. No. 5,814,349 to Geus et al discloses a device which combines quench fluid flow with below the belt suction. However, this arrangement requires a decoupling device in order to prevent skein forming deceleration which negates the original advantages of the U.S. Pat. No. 5,032,329 to Reifenhauser.
Polypropylene is the only thermoplastic resin that is commonly utilized in conventional air drawn spunbond processes. It is important to note that due to the limitations of existing spunbond spinning systems it is virtually impossible to process resin entities in equipment designed for polypropylene where flow and spinning characteristics deviate significantly from polypropylene.
As a first step, the resin is melted and extruded through a spinneret to form a vertically oriented cascade of downwardly advancing molten fibers. The filaments are fluid cooled to quench and uniformly cool the filament curtains for optimum drawing and development of the desired high crystallinity which provides the goal of high fiber strength. A fiber drawing system having a fluid draw jet-slot, into which a controlled volume of high velocity fluid is introduced, draws additional fluid into the upper open end of the drawing slot and creates a rapidly moving downstream of fluid within the slot. This fluid stream creates a contiguous drawing force on the filaments, causing them to be attenuated. After the filaments are attenuated they exit the bottom of the slot where they are deposited on a moving conveyor belt to form a continuous web of the filaments. The filaments of the web are then joined to each other through conventional calendering and point bonding techniques.
Forming filaments in the well known conventional spunbond systems results typically in filaments of 2.5 denier to 12 denier and higher. Using conventional methods, the molten filaments leaving the spinneret typically are immediately cooled at their surfaces to ambient temperature and then subjected to the typical drawing system. This conventional method and apparatus produce adequate non-woven fabrics however their properties, especially tensile strength, high machine direction to cross direction strength ratio, non-chemically enhanced hydrophobicity, drape, softness and opacity are poor.
When conventional spunbond systems attempt to make sub-denier fiber the resin output per hole drops precipitously reducing spunbond fabric production to less than half of the production when forming spunbond of typical denier range.
The instant invention through the use of a unique new apparatus and process, provides a greatly improved spunbond fabric consisting of a narrow range of low denier filaments which improves all of the aforementioned properties.
The low-denier filaments with their smaller diameter produces more surface area and more length per unit weight, reduces light transmission and improves light dispersion (greater opacity) and softness (lower unit fiber deflection forces). Using the instant invention spunbonded fabrics can be made from a wide range of resins, in addition to polypropylene, such as polyethylene, polyester, polyamides, polycarbonate, polyphenylene sulfide, liquid crystal polymers, fluropolymers, polysulfone and their copolymers as well as other extrudable synthetic resins. Providing narrow ranges of filament sizes from 0.1 denier to 1.0 denier with a wide range of polymers is extremely desirable because of their improved performance properties as indicated above. A further process benefit of the instant invention is that resin throughput per hole per minute is not reduced below existing commercial rates.
Examples of end uses for the instant invention are filtration materials, diaper covers and medical and personal hygiene products requiring liquid and particulate barriers that are breathable and provide good vapor transport with significant air permeability. Because of the low denier the spunbond fabrics produced by the instant invention have physical and performance properties comparable to SMS (Spunbond-Meltblown-Spunbond), SMMS (Spunbond-Meltblown-Meltblown-Spunbond) and SM (Spunbond-Meltblown) fabrics. This is an important result since it suggests that a single die head or beam can produce a material which now requires from two to four die beams.
Prior conventional spunbond art is almost completely concerned with the use of polypropylene. An important limitation of prior art is the inadequacy of conventional spunbond systems to extrude and highly draw common resins such as polyester, polyethylene or more unusual resins such as polyamides, polycarbonate, polysulfone and polytetrafluoroethylene.
The instant invention teaches apparatus and processes that are designed with intrinsic accommodations to extrude and draw fibers with an extreme range of extrusion temperatures, wide variations in glass transition temperatures, wide ranges of melt viscosities and other variable resin properties important to filament extrusion, forming, quenching and drawing, thereby widening the application of the spunbond arts.
It is the principal object of the present invention to provide an improved system for the production of spunbonded nonwoven webs of thermoplastic synthetic resin filament which allows:
1. significant increase in filament velocity and attenuation over a wide range of filament diameters.
2. significant decrease in fiber denier or diameter at lower operating costs without sacrificing mass through-put.
3. capability of spunbonding a wide variety of resins using one apparatus having a wide degree of adjustment in the extrusion, forming, quenching, drawing and laydown operations.
4. stronger fibers through improved crystallization kinetics based on improved attenuation and quench control.
5. higher nonwoven fabric opacity and cover.
6. increased fiber and nonwoven fabric uniformity (narrower filament diameter range).
7. significant increase in collector speeds with resultant higher mass throughput.
8. production of webs with filament deniers of less than 1.0.
9. production of light weight webs at collector speeds in excess of 600 meters per minute.
10. production of nonwoven material at mass rates of greater than 400 to 600 kg/hr/meter of die width.
11. Filament spinning speed of greater than or equal to 7000 meters/minute.
More specifically, it is an object of the invention to improve a spun-bond apparatus so that the throughput of the synthetic resin filament is increased and the production rate enhanced without encountering drawbacks typically found in spunbond apparatus such as excessive energy consumption and poor web uniformity.
Other important objects of the invention are to provide:
1. an improved method of operating a spunbond apparatus to eliminate drawbacks thereof by increasing the degree of attenuation while decreasing the filament denier at relatively low energy cost with a minimum of process complexity.
2. an improved method of feeding precise amounts of resin to each orifice in the spinnerets using multiple feeding mechanisms.
3. an improved filament extrusion die with the capability of containing a greater number of extrusion orifices per meter of die width and length. an improved apparatus for the purposes described which allows the operating conditions within the apparatus to be varied in a sufficiently wide range of relationships to accommodate a large variety of resin materials and for the production of a wide range of products without the limitations characterizing earlier and present spunbond production systems.
4. improved quenching performance and uniformity by precise control of fluid temperature and velocity in a plurality of descending zones of the quench fluid system.
5. an improved apparatus including a fluid inlet infuser, a draw jet-slot, a draw jet-nozzle, a venturis, and a outlet fluid diffuser which are independently adjustable to provide optimum process control over a broad range of resins.
6. an improved apparatus and process which increases the drag force on fibers by inducing a controlled sinusoidal fiber track which permits the fiber velocity to be increased by increasing the area of fiber exposed to the drafting fluid drag forces thus significantly reducing the filament denier and decreasing energy requirements.
7. an improved apparatus and process which provides controls induction of fluid into the draw jet slot extension below the venturi to induce mini-vortices at the walls and provide a turbulent boundary layer.
8. improved uniformity of filament laydown by controlled turbulent separation of the fiber cascade at the entrance to the lower adjustable fluid volume control diffuser.
9. an improved method of making nonwoven webs of synthetic resin filaments whereby drawbacks of earlier and present conventional spunbond systems, especially limitations on draw force, fiber velocity, fiber formation and web collector speed are eliminated.
All of the aforementioned process and product improvements are an integral result of the system which is presented below.
An apparatus for the production of sub-denier spunbonded nonwoven fabrics has, according to the invention, a resin extrusion device, a unique multi-head metering system for micro-metering resin to micro-distributors in the spinnerets, a spinneret die head with dual front and back perforated spinning sections, separated by a buffer section or quench fluid extraction zone having a lower density of perforations and in some embodiments no perforations, wherein the buffer section allows full and uniform penetration of quench fluid, for extruding a multiplicity of continuous thermoplastic strands that then descend through a two sided, multilevel quench system and thence through a fluid volume control infuser system, which meters quench fluid into or, if required by the process conditions, out of the filament drawing system.
The quench fluid is supplied from a blower through one or more heat exchangers into a controlled three level manifold which permits flow rate and temperatures to be controlled independently into each segment of the quench cabinet.
The dual spinning sections with the unique buffer zone or quench fluid extraction zone located between the two outside spinning sections is a very important part of the instant invention because it permits the use of more spinneret orifices per meter of width than can be accomplished in conventional systems. This is accomplished by using a high density of orifices in the two outside spinning sections and a central fluid buffer zone or quench fluid extraction zone located between the two outside spinning sections. Experimentation with the design of the buffer zone indicated that it could also be used for the production of additional filaments without creating a disturbance in the filaments at the point of the two streams"" impingement. We further found when the filament density, or orifice density, was about eighty percent or less of the filament density of the dual spinning sections that impingement of the opposing fluid streams in the buffer zone was not an issue. Consequently the central buffer zone may contain a reduced density of perforations, or in some embodiments, a zero density of perforations.
This overcomes the necessity to significantly reduce resin flow per hole per minute which is the main drawback in producing low or sub-denier fibers at commercially acceptable rates. The end result of the flow reduction is that low denier fiber production is always reduced far below commercial expectations. Furthermore, inadequate control of the quench process results in ineffective drawing with resultant non-uniform and weak fibers.
The bilateral nature of the split array orifice spinnerets with an independently controlled bilateral quench system also permits the use of two different but compatible resins, one on each side, or a differentially quenched bicomponent filament.
The filament cascade is automatically guided into the filament drawing system by the fluid volume control infuser system which depends from the lower surface of the quench assembly and is extensibly attached to the draw jet assembly. The purpose of the fluid volume control infuser system is to conserve energy by using a portion of the quench fluid as part of the drawing fluid and simultaneously minimizing turbulence at the entrance to the draw slot thus providing a uniform cascade of filaments to the drawing step. This arrangement provides a self feeding action for the descending cascade of filaments and is extremely important from an operational standpoint.
The fluid volume control infuser system consists of two perforated plates oppositely situated and variable, as to angle, open area and vertical length, each containing a multiplicity of uniquely shaped and oriented perforations to permit two-way fluid flow. Further, the open area of the multiplicity of fluid holes is controllable as to area by use of a slide gate or similar fluid volume control means. The holes or amount of open area controls the amount and pressure of fluid in the infuser and controls turbulence but allows the fluid to be automatically bled off or entrained.
When quench fluid, descending from buffer zone, is drawn into the fluid volume control system infuser by its downward velocity and the suction developed at the inlet of the draw jet slot opening by the draw jet flow an over-pressure condition may occur which may cause turbulence at the slot inlet. The combination of the fluid scoop shape and the open area of the infuser plates permits the automatic shedding of excess fluid and the balancing of pressures as the fluid and filament velocities increase into the slot. The variable area permits the specific adjustment for different resin species where the quench fluid may be very high or low in volume and velocity. The major axis length of the perforated holes ranges from 10 millimeters to 100 millimeters. Each row may have different sized holes. The fluid scoop portion of the hole is elevated above the outer surface of the infuser plate.
The infuser plates have a sliding means in their lower portion which permits the distance between the lower edge of the quench system and the upper surface of the draw jet assembly to be adjusted to required process conditions for different resin species.
The filament drawing system consists of a draw jet assembly that contains a variable width draw jet-slot and variable width draw jet-nozzle. The assembly consists of a right and a left hand vertical halves. The right and left hand vertical halves are moveable horizontally in relation to each other. The entire draw jet assembly is moveable vertically in order to optimize the distance between the draw jet-slot and the emerging filaments at the spinnerets.
The space between the left and right vertical halves defines the variable width slot used to vary drawing velocity. The upper surface of both the right and left hand halves of the assembly contains an adjustable nozzle plate that is moveable horizontally in relation to the slot wall and serves to define the variable width draw jet-nozzle outlet passage and thus adjusts the draw jet fluid velocity. The angle formed by the centerline of the primary jet-nozzle and the centerline of the draw jet-slot ranges from 2 degrees to 45 degrees. The slot extends vertically to the draw jet extension and horizontally the width of the spinneret head. The draw jet-nozzles formed by the adjustable nozzle plate and the upper edge of the vertical halves provide motive fluid for the drawing process, extend the full horizontal width of the jet-slot.
Experimentation showed that when the two horizontally opposed and adjustable draw jet-nozzles are offset vertically by a centerline distance of from 1 millimeter to 50 millimeters the draw force is still very high but, surprisingly, a vertical sinusoidal oscillation is created in the descending cascade of filaments. The filaments produced with this innovation were significantly finer than when the jet-nozzles were directly opposed and not offset. The oscillation produces a higher filament drag coefficient and thus increase the energy transfer coefficient between the filaments and the draw jet fluid stream thereby increasing the fiber attenuation.
Further experimentation showed that this oscillation could also be produced by several alternative methods. When a second set of adjustable gap jet-nozzles are located in the slot wall on each side of the left and right hand assembly halves and below the primary draw jet-nozzles, and when these secondary jet-nozzles are directly opposed and not offset, and are provided with a system that emits pulses of fluid at a fixed angle across the slot alternately from each side these secondary jet-nozzles also create a small sinusoidal oscillation in the filament cascade which provides a larger drag area for the motive fluid to impact and to accelerate the individual filaments. The angle formed by the center line of the secondary jet-nozzles and the centerline of the draw jet-slot ranges from 2 degrees to 45 degrees. The increased drag coefficient also provides a more efficient transfer of energy to the filaments. The secondary jet-nozzle may also suck fluid out of the draw jet-slot in the same alternating pulsation mode. It was also discovered that off-set pulsating jets also produced the required oscillations.
Experimentation has also shown that the filaments may also be oscillated by a constant or intermittent flow from only one side. It was eventually discovered that the secondary jet-nozzle system worked best when they were offset and the flow was constant from each side. It was discovered that in the primary jet plus secondary jet configuration the additional fluid flow together with improved drag factor from the oscillation effect added an unexpectedly high velocity increment to the filament curtain which resulted in remarkably low fiber diameters which were in the 0.5 denier to 1.2 denier range depending on the system configuration. Adjustable gap secondary draw jet-nozzles were also evaluated and determined to provide even better control of denier. Both the primary and secondary jets are preceded by a full die width pressure equalization and distribution system.
Below and attached to the lower half of the draw jet assembly is a supplemental acceleration device or draw jet slot extension, which has a horizontally adjustable slot similar to the draw jet assembly slot but which is also vertically adjustable and contains two in-line or tandem venturis or other fluid acceleration devices to maintain fiber tension and draw force through the lower end of the draw system. Alternative fluid acceleration devices such as a NASA profile convergent-divergent nozzle or other fluid acceleration means can also be used.
The draw jet extension has an adjustable slot and venturi width to control draw velocity and maintain constant tension on the filament cascade. The draw jet extension""s distance above the foraminous collector belt is also adjustable.
Below each venturi is an additional set of adjustable inlet jets on both sides which may be used to suck in ambient fluid thereby creating a series of micro-vortices in the wall boundary layer. This creates a turbulence at the wall between the first venturi and the second venturi and after the second venturi prior to the exit into the fluid volume control diffuser system.
The fluid volume control diffuser system consists of two perforated plates oppositely situated and variable, as to angle, open area and vertical length. The major axis length of the perforated holes ranges from 10 millimeters to 100 millimeters. Each row may have holes with different major and minor axis length. The fluid scoop portion of the hole is elevated above the surface of the diffuser plate. The plates depend from the bottom of the draw jet-slot extension assembly and which lower adjustable ends may be abutted to vacuum seal rollers or other sealing means, or open to the atmosphere.
In the case where the plates are open to the ambient atmosphere the ends of the plates are adjusted to the correct distance above the foraminous belt. The distance of the two plate ends above the foraminous belt may be equal or unequal.
Generally in the case where the ends of the plates are open to the ambient atmosphere the deposition of fibers is more uniform if the longer plate is on the up stream side in reference to the belt travel direction.
These plates contain a multiplicity of fluid holes which are controllable as to total area by the use of a slide gate or other means. The holes or amount of open area controls the amount and pressure of fluid in the diffuser and controls turbulence but allows the ambient fluid to be automatically entrained. This has a beneficial effect on the uniformity of filament lay down by controlling the rate of deceleration of the filaments.
The filaments begin to decelerate upon entry into the fluid control system and begin to describe a downward spiraling motion which assists in developing a uniformly isotropic web deposited on the foraminous conveyor belt used to receive and convey away the web. The fluid volume control system is adjustable as to the diffuser angle and open area.
When the included angle between the two halves is wide the swirl approaches an elliptical appearance with the longer axis in the machine direction. Narrowing the included angle shifts the elliptical pattern to the cross direction. Proper angle and fluid flow adjustment of the fluid volume control diffuser is based on belt speed and required areal web weight so that the resultant swirl pattern on the moving belt is most nearly circular. A circular pattern provides the most isotropic product physical characteristics wherein the machine to cross direction ratios of physical properties such as tensile strength and elongation approach a ratio of 1:1. This is significantly better than typical spunbond fabrics which generally have ratios in the 2:1 or higher range especially at low areal weights and high belt speeds. The narrower ratio permits lighter weight fabrics to be safely used in applications such as disposable diapers where cross direction tensile strength is an important consideration from both the diaper manufacturing and end use requirements.
In order to maintain complete and total control of the system fluid and also reduce the load on the under belt suction device it is necessary to prevent the incursion of ambient fluid into the space between the outlet of the diffuser system and the belt as well as between the belt and the plenum.
This is accomplished by creating a sealing system where the lower end of each fluid volume control diffuser system plate assembly is affixed to a curved surface which is slidingly adjoined to a set of upper vacuum seal rolls. This effectively seals the control system against fluid being sucked in at the lower edges of the volume control system thus minimizing any possible turbulence which might interfere with filament lay down. The curved surface is designed such that surface is continually in sliding contact with the surface of the stationary vacuum seal rolls. regardless of the angle of the diffuser system. The curved surface or shoe is covered with a replaceable low pile fabric to aid in sealing. Alternatively the rolls may be covered with fabric.
The two above the belt sealing rolls are paired with two below the belt sealing rolls in order to provide an essentially leak proof connection between the diffuser ends and the upper opening to the vacuum plenum. The lower sealing rolls are also slidingly sealed to the plenum. The lower or suction opening of the vacuum plenum is connected to a variable volume suction blower or other variable volume suction pressure device by a duct.
To decrease the web thickness prior to the deposition of an additional web or the web bonding step it is compacted by a driven web compaction roll set directly after leaving the vacuum area.
The variable speed foraminous collector screen or belt then delivers the web or multiple webs to a filament bonding station, such as thermal pattern bonding or other means of web bonding or interlocking.
It is anticipated that this unique spunbond system will be used in combination with a meltblown system and a second unique spunbond system to provide a unique in-situ three web laminate. It is further anticipated that this unique spunbond system will be used in combination with a meltblown system to provide a unique in-situ two web laminate.
It is further anticipated that using the instant invention, spunbond fabrics with average filament sizes below 0.7 denier will have, opacity, resistance to liquid penetration and other physical and performance properties comparable to SMS webs.
Glossary of Terms
In order to better understand the terminology used herein, particularly those terms which may be ambiguous with respect to some prior art or which have been indiscriminately used without explanation in the prior art, the following definitions are submitted.
Aspirate: to draw by suction
Aspirative means: a means by which an internal force such as a suction or differential pressure sucks or draws fibers or fluid through a passage or slot
Buffer zone: see quench fluid extraction zone
Capillary: refers to the resin extrusion orifice or any other drilled hole or perforation that serves as an orifice
Crystallinity: the relative fraction of highly ordered molecular structure regions compared to the poorly ordered amorphous regions as determined by X-ray or other appropriate analytical means
Die head: refers to complete structure containing the spinnerets, resin distributors and other associated filament extrusion equipment and which extends across the full width of the spunbond machine, also referred to as a die beam
Diffuser: a diverging channel transition system for controlled reduction of the velocity of the fluid and filaments exiting the filament drawing system and entering the filament lay-down system
Educt: to draw out
Eductive means: a means by which an external force such as a suction fan creates a differential pressure that draws fibers or fluid out through a passage or slot
Fluid volume control plate open area: the ratio of the actual area of the holes as precluded by the slide control plate to the total area of the fluid-scoop holes
Induct: to bring in
Inductive means: a means by which an external force such as a pressure fan creates a differential pressure that transports or brings fibers or fluid into or through a passage or slot
Infuser: a converging channel transition system for controlled funneling of fluid and filaments into the filament drawing system
Jet: a slot, nozzle, perforation or other orifice through which a fluid may be emitted or drawn in and which may have an opening that is round, rectangular, or any other shape without regard to length or diameter
MD/CD ratio: ratio of a fabrics machine direction to cross direction properties typically used as a measure of isotropic formation
Quench fluid extraction zone: That portion of the area between the quench cabinets where the bilateral quench fluid streams meet and descend into the fluid volume control infuser
Resin: refers to any type of material that may be liquefied to form fibers or nonwoven webs including, without limitation, polymers, copolymers, thermoplastic resins, waxes, emulsions and the like